This proposal focuses on cytoskeletal mechanisms necessary for normal mitotic exit: cytokinesis and spindle disassembly. The proposal builds on three series of novel findings. First, in budding yeast we discovered a mechanism by which the Polo kinase (Cdc5) controls cytokinesis. In late mitosis, Cdc5/Polo targets guanine nucleotide exchange factors (GEFs) for the small G-protein Rho1 (human RhoA) to the division site. The GEFs, in turn, recruit the small G-protein Rho1. Rho1 targeting and activation are essential for the recruitment of formins, actin nucleators required for contractile actin ring (CAR) assembly in all eukaryotes. Similar mechanisms were recently shown to explain the role of human Polo-like kinase in CAR assembly in human cells. Second, we have unpublished evidence that the yeast formin Bni1 is regulated by a novel mechanism involving regulated proteolysis. Third, we have defined the biochemical properties of a key regulator of spindle disassembly. Kip3, a member of the Kinesin 8 family of proteins, is both a plus end directed MT motor and a plus end-specific MT depolymerase. We defined new mechanisms regulating the Kip3 depolymerase activity which may be central to controlling spindle dynamics during late mitosis. We now propose experiments to (1) define the mechanism of Rho1 targeting to the division site and the mechanism controlling global Rho1 activation during mitotic exit;(2) Define the mechanisms and biological role of regulated formin degradation;(3) Characterize novel Kip3/Kinesin 8 regulatory mechanisms and define their role in spindle disassembly and microtubule dynamics. The proposed research is relevant to cancer biology because cytokinesis failure is known to promote tumorigenesis. PUBLIC HEALTH RELEVANCE: This proposal will define mechanisms that mediate changes in the cytoskeleton necessary for mitotic exit: cytokinesis and disassembly of the mitotic spindle. The results will be relevant to cancer biology because abnormal cytokinesis is known to promote tumorigenesis.